Question

Solve for r: C = 2r

Answer 1

So no numbers? If not, they mean multiply both sides by a half.

Answer 2

pie is in front of the r

Answer 3

When they do not give numbers, you use the same rules as when they do. You just multiply, divide, add, and subtract as needed until whatever they say to solve for is alone.

Well, then divide it out too.

Answer 4

It is just like any other algebra problem. \(\pi\) is just another number. Nothing special. Just do the same things you would always do:
\[C=2\pi r\implies \\
\frac{C}{2\pi}=\frac{2\pi r}{2\pi}\implies \\
\frac{C}{2\pi}=r\]